SPI: create a transation manager

create a manager to all transactions to validate the step, the pacs and the time before drop it, should save the PSPs and when expired, call an callback

No files, tests, or entry points are named. Start by locating the existing SPI transaction flow and clarify how steps, PACS, timeouts, PSP persistence, and expiration callbacks are represented. Done should mean the requested transaction validation and expiration behavior is covered by project tests.

Written by the indexing model from the issue text.